In food shops in convenience stores, department stores, super markets, etc., foods, such as boiled rice, daily dishes, noodles, salads and processed meat, e.g., ham are placed in food containers, e.g., tray, cup or bowl, and sold. Such a food container is composed of a container body and a cover. The container body is, in general, manufactured by thermoforming a sheet of polypropylene, foamed polypropylene, filler-containing polypropylene, polyethylene, foamed polyethylene, formed polystyrene, foamed heat-resistant polystyrene, amorphous polyethylene terephthalate (A-PET), etc, by a vacuum forming machine, a pressure forming machine or a vacuum-pressure forming machine. The cover is formed from a sheet, such as A-PET, biaxially oriented polystyrene (OPS) or polypropylene (PP) (JP2005-329972A).
Recently, it is frequently conducted that foods packaged in a food container are heated as it is by a microwave oven. When foods are heated together with the food container by a microwave oven, temperature of the foods is raised to around 100° C. Accordingly, food containers are required to have a heat resistance resisting up to around 100° C. Food containers for retort foods are required to resist a retort sterilization temperature at 125° C. Furthermore, food containers are desirably to have high transparency so that the foods packaged therein can be appreciated clearly at a look and can improve commercial value.
However, none of the above-mentioned conventional sheets satisfies both of high heat resistance and high transparency. That is, A-PET and OPS have high transparency, but have not high heat resistance and are softened at around 70° C. PP sheet has high heat resistance but is inferior in transparency.
Thereupon, the applicant devised a method of making a heat-resistant transparent container which comprises a primary stretching and heat-setting process wherein an amorphous polyethylene terephthalate sheet is heated, primarily stretched and then primarily heat-set, and a secondary stretching and heat-setting process wherein the sheet treated in the primary stretching and heat-setting process is molded with heating in a mold of a thermoforming machine while secondary stretching is performed followed by secondary heat-setting in the same mold (JP 4223520 B2, EP 1870226 A2).
In the meantime, daily delivering foods having use-by date within 2 days, cold-storage foods having relatively long-term use-by date, retort foods having long-term use-by date, and the like are sold in food shops in convenience stores and super markets, and food containers are required to have an ability to keep quality of foods corresponding to the use-by date.
However, although the above heat-resistant transparent container developed by the applicant is very favorable which has high heat resistance and high transparency, it is not sufficient in terms of ability to keep quality of foods. In addition, its resistance to dropping is also required to be improved.